1. Field of the Invention
The present invention relates to a light scanning apparatus and an image forming apparatus.
2. Description of the Related Art
In recent years, in the image forming apparatus such as a laser beam printer or a digital copying machine, a so-called overfilled optical system, in which light flux having a larger width in a main scanning direction than a reflection face of a polygon mirror (rotating polygon mirror) of a light deflector in incident on the polygon mirror, dimensions of the reflection face is decreased, and the number of reflection faces is increased to be scanned, has been realized in accordance with a demand for higher speed and density of the image formation. However, in a scanning optical system using the overfilled optical system, because the scanning is performed by using a light beam of which light quantity, typified by Gaussian distribution, varies according to a position of a face being scanned, there is a problem that uniformity of a light quantity distribution is lost. For example, the intensity is high in a central portion of a main scanning line, but the intensity is low in an end portion, and the like.
In order to solve the problem, Japanese Patent Application Laid-Open (JP-A) No. 11-218702 (Patent Reference 1) has proposed that ununiformity of the light quantity on the face being scanned is corrected by using an aperture in which the width in a direction intersecting a scanning direction is changed in the scanning direction.
FIGS. 13 and 14 show a structure and a principle of a light scanning apparatus described in JP-A No. 11-218702 (Patent Reference 1). In the light scanning apparatus, the light beam irradiated from a light source 12 is condensed by a collimator lens 14, passed through an aperture 17 in which the width in the direction intersecting a rotating direction of a polygon mirror 26 is changed, and focused on a face being scanned 32 through an enlarging optical device 18, the polygon mirror 26, a scanning optical system 24, and a folding mirror 30.
At this point, as shown in FIG. 14, the light beam having light quantity as the Gaussian distribution is made into a state equal to a light beam of which light quantity exhibits a combination of the Gaussian distributions having different peaks, by the aperture 17 in which the width in the direction corresponding to a sub-scanning direction is changed. A part of the light beam whose light light quantity is transformed is reflected by a reflection face 28 of the polygon mirror 26 to be focused on a medium being scanned. Consequently, the light quantities of imaging spots of different positions on the medium being scanned are substantially the same, and the uniform light quantity distribution is formed, as shown in FIG. 15.
Thus, the structure described in JP-A No. 11-218702 (Patent Reference 1) has an effect that a deterioration of the uniformity of the light quantity, which is caused by the overfilled optical system, particularly, the deterioration of the uniformity of the light quantity, which is caused by a decrease in illuminance in the position away from an optical axis (outside axis), can be corrected. However, there is caused a problem that sensitivity for the change in the light quantity distribution is decreased on the face being scanned. For example, when a laser beam has an asymmetric light quantity profile, sometimes alignment of the light beam incident to the rotating polygon mirror or the position in the scanning direction of the aperture portion is adjusted in order to secure the uniformity of the light quantity, in such case, the large amount of the adjustment is required in adjusting. Therefore, a light path is located on the position that is shifted from the design light path, which results in a problem that a beam diameter is enlarged on the face being scanned or the uniformity of the beam diameter is lost.
On the other hand, JP-A No. 9-96769 (Patent Reference 2) discloses an adjusting method, comprising the steps of: detecting a light quantity by a plurality of detectors provided in inside or outside of the light scanning apparatus in a light scanning apparatus using the overfilled optical system; and adjusting the optical axis of a light source apparatus so that the light quantity distribution is uniformed on the basis of a detection value of the light quantity.
In the adjusting method, as shown in FIG. 16, a light source 24 is moved and adjusted so that the intensity of the light beam incident to a light quantity sensor 81 of a scanning start side is substantially equal to the intensity of the light beam incident to a light quantity sensor 84 of a scanning finish side, and the alignment of the light beam is adjusted. Accordingly, a maximum intensity portion of the intensity profile of the laser beam can be corresponded to the optical axis of the optical system, the light quantity of the scanning start end and that of the scanning finish end can be uniformed without losing the uniformity of the beam diameter on the face being scanned. However, the deterioration of the uniformity of the light quantity, caused by the overfilled optical system, and the decrease in the light quantity in both scanning ends, caused by the decrease in the illuminance in the outside axis, can not be prevented.
On the other hand, JP-A No. 8-171069 (Patent Reference 3) discloses that a decrease in light transmittance of the whole optical system and deterioration of the uniformity of the light quantity are suppressed within a certain range, so that such a decrease and deterioration cause no problem in practice, by setting the width of the light flux incident on the reflection face or surface (i.e., the width in the rotating direction of the rotating polygon mirror) 1.5 to 4 times as much as the width of the reflection surface.
However, in the method described in JP-A No. 8-171069 (Patent Reference 3), several problems occurs in the case that the method is adopted for an image forming apparatus in which the higher image quality is demanded.
First, for example, in the case of D0/Fa=1.5, the light flux width on the polygon mirror can not completely cover a deflecting reflection face at a maximum scanning angle, the decrease in the light quantity occurs at both ends of the scanning even in an effective scanning region, and density is decreased.
In the case of D0/Fa=4, the light having the large intensity is also incident on, among a plurality of the reflecting surfaces of the polygon mirror, a reflection face (an adjacent reflection face) adjacent to the reflection face which scans and deflects the light beam. The unnecessary light reflected by the adjacent reflection face is not directly incident to an image region of a recording medium which is being scanned. However, when unnecessary light is reflected by an end face of a lens and the like in the image forming apparatus, the unnecessary light reaches the recording medium with the sufficient intensity enough to expose the medium which is being scanned, and becomes a so-called flare light. The flare light generates a ghost on the face being scanned, and degrades the image quality. Accordingly, new light blocking means is required in order to block the flare light.
Further, as shown in FIG. 17, even if the ratio D0/Fa of the light flux width D0 on the polygon mirror to the scanning direction width Fa of the reflection face of the polygon mirror is set to 4, a ratio of the light quantity at the scanning start position to that at the central position of the scanning does not exceed 90%, and the light quantity of the end portion is decreased by 12%, as compared with the light quantity of the central portion. The amount of the decrease depends on the design of the image forming apparatus. For example, in the image forming apparatus such as a color printer using a lot of half tones, the change in the density is recognizable by eyes, and thus could be a relatively a large obstacle in achieving high quality image.
In order to prevent the decrease in the light quantity, it is thought that the light quantity distribution is corrected by the aperture portion 17 described in JP-A No. 11-218702 (Patent Reference 1).
However, when the light quantity distribution (uniformity) is corrected by the aperture portion 17 described in Patent Reference 1, since the amount of change in alignment for correcting the uniformity of the light quantity caused by the above-described asymmetric profile is increased, the shift of the light path is further increased, and the deterioration of the beam diameter uniformity becomes remarkable.
Thus, in the structure in which the ratio D0/Fa of the light flux width D0 on the polygon mirror to the scanning direction width Fa of the reflection face of the polygon mirror is set in the range of 1.5 to 4, when the structure in which the light beam is passed through the aperture portion is formed so as to correct the ununiformity of the light quantity, a new problem inevitably occurs.